The present invention relates to improvements in methods and mechanisms for dewatering fibrous webs and particularly to an improved method and mechanism for forming a web from bagasse fibers.
In the formation of a paper web at high speeds, it is customary to deposit a slurry on a traveling porous forming surface to obtain initial dewatering and formation of the web on the forming surface. The web is then transferred customarily to a press section where additional water is squeezed from the web and then transferred onto a dryer section.
Developments have been made in the art for many years in attempts to improve dewatering of webs in the press section, and to improve reliable handling of webs as they travel through the machine without breaking. Examples of such attempts are disclosed in U.S. Pat. Nos. 3,600,273, 3,600,275, 3,671,389, 2,694,346, 2,850,951, Canadian Pat. Nos. 434,032 and 773,352 and other patents and developments are known to those versed in the art.
It has been discovered that the formation of a web from fibers of bagasse present unique problems not encountered with the formation of web from wood fibers at ordinary speeds. To make a paper web from bagasse fibers, the web must be satisfactorily formed and handled at commercially useful rates of web travel. Also, to make useful web such as for newsprint, the bagasse web must possess the qualities necessary for finished newsprint including adequate ink receptivity, uniformity or one-sidedness so that both sides of the web can be printed on and fiber formation which provides adequate strength and has not been crushed during dewatering. The desirability of providing a successful mechanism and method for handling bagasse is required because of the ready availability of bagasse fibers in certain geographical areas and such fibers being relatively inexpensive.
It has been discovered that among the problems presented by bagasse fibers is that the web in its wet state is relatively weak so that substantial difficulties are encountered in attempting to handle the web, particularly at good speeds. It has been discovered that the wet web is particularly weak until dryness in excess of 30% are reached.
Another difficulty which has been discovered is that bagasse shows a marked tendency to pick on smooth surfaces particularly when wet. Picking is caused by the fibers clinging to traveling surfaces and being pulled from the surface of the web so that the web is spoiled, and the fibers which remain on the machine surfaces will tend to spoil the succeeding traveling web passing through the machine. In handling the bagasse web to bring it to a state of dryness to increase its strength and decrease its tendency to pick, the dewatering must be accomplished in such a manner so that the water is not brought out of the web too rapidly so as to cause crushing. Crushing is a phenomena which occurs in accordance with certain theories by the water being forced out of the web at too high a speed so that internal hydraulic pressures build up to dislodge or dislocate the individual fibers and destroy their orientation and bond. If crushing occurs, the resultant web will be weakened and of poor quality.
Various press structures employ a plain roll, and a fibrous web is laid on the plain roll and successive press rolls are arranged around the plain roll to form successive nips with a felt being passed between the web and the clustered rolls. Advantages are obtained in this type of pressing arrangement in that the web can be successfully pressed without having to be handled in between nips and the water is pressed out of the web successively in the same direction to pass into the felts. However, with a bagasse web, because of the tendency to pick and the weakness of the web, it has been impossible to take advantage of the features of this type of press. The present invention permits using a method and structure wherein such a plain roll press can be employed and coacts with preceding press nips to be able to handle the web and improve the actual operation of the plain roll press by bringing the water level in the web to an amount where its action is more effective and wherein the migration of water in a single direction out of the web can be used to advantage for better and more effective dewatering to improve the quality of the web and place it in a condition of having less moisture when it is passed onto the dryer section of the machine.
In passing the web through the nips formed against the plain roll, improved dewatering is attained without picking, and one theory of operation accounting for this improved dewatering, and by which applicants do not necessarily wish to be bound is that in accordance with the teachings herein contained, the web has reached sufficient dryness so that the water can exit from the web in a single direction at rates caused by relatively high nip pressures, without crushing. Thus, in the third nip, which is the first nip of the plain press roll, water is brought out of the web into the felt and additional water is brought to the outer surface of the web. Having brought the water in this direction toward the surface of the web in the third nip, it exits more effectively and more efficiently in the same direction in the fourth nip. Also, since no water has passed inwardly toward the plain roll, the web is in effect maintained smooth with the fibers against the plain roll remaining lubricated with moisture to reduce the tendency to pick and the fibers will separate easily from the plain roll when the web is removed therefrom.
It is accordingly an object of the invention to provide a method and mechanism which is capable of controlling a wet web of bagasse in such a manner that it is completely supported and controlled during the initial pressing dewatering stages until it reaches a dryness of at least 30% and applying dewatering pressing forces in such a manner that the integrity of the web is protected and crushing does not occur, and that the tendency of the fibers to pick are avoided. A general object of the invention is to provide a method and mechanism which balances the need for rapid and effective dewatering with the need for control and support of a web of relatively weak wet strength so that an improved web results, yet the dewatering operation can take place at satisfactory commercial operating speeds.
A still further object of the invention is to provide a method of dewatering traveling webs of bagasse fibers in such a manner that a uniform web of superior inking quality on both sides results and is particularly well suited to the relatively high speed manufacture of bagasse newsprint.
A feature of the invention is the support of the web on the top surface of a felt almost immediately after it is picked off the forming surface. The web is kept under complete control by being picked off the forming surface by suction pick-off roll which provides no opportunity for the web to break loose and travel downwardly and thereafter the web is transferred onto the upper surface of a traveling press felt and maintained on the top surface of that felt continually through two double felted nips. The web does not depend upon any frictional forces or pressure differential forces to cause it to adhere to the undersurface of a felt, and thus there is no opportunity for any surface fibers on the web to become dislodged or displaced nor for the web fibers in any way to separate due to the weight of the web, nor for the web fibers in any way to separate due to the fact that such web must be carried by being supported along the upper surface of the web as is the case with certain structures heretofore available. By being supported on the top or outer surface of a press felt, the web is gravitationally held in place, and there is no possibility whatsoever of the web being lost or being broken, and the web strength and the relative positions of the fibers are maintained intact at a very critical time in its formation when the moisture level is such that the fibers are attracted to each other, but with insufficient force to maintain web integrity by being handled in the usual manner. A further feature of this arrangement is that the application of pressing pressure is applied at a gently increasing increment due to the twin double felted nips to cause the water to travel in both directions out of the web, but also to cause the water to move out of the web at a gently accelerating rate. While some rewetting occurs on the offrunning side of the felt nips, the rewetting also transpires in a gentle manner so that any tendency to upset the orientation of the fibers is reduced. These features of processing and handling accrue to particular advantage in the formation of a bagasse web, but may also be employed to overcome difficulties in forming a wood fiber web at very high speeds in excess of those which are now commercially feasible.
Other objects and advantages and features will become more apparent, as will equivalent methods and structures which are intended to be covered herein, with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiments in the specification, claims and drawings in which: